Field of the Invention
The present invention concerns a method and a magnetic resonance system in order to determine a resonance frequency deviation namely an off-resonance (frequency), which occurs in the event of magnetic field inhomogeneities.
Description of the Prior Art
If a patient has a metallic implant and MR images of a volume segment in proximity to this implant are created, severe artifacts occur within these MR images. These artifacts are based for the most part on inhomogeneities of the basic magnetic field which in turn arise due to strong susceptibility differences between the metal of the implant and the surrounding tissue. A reliable diagnosis by a physician is in most cases is impossible based on MR images that exhibit these artifacts.
Among other things, inhomogeneities of the basic magnetic field have the effect that an excited slice in the slice selection direction is spatially distorted. The distortion or geometric shift of the slice that occurs due to the inhomogeneities is thereby proportional to the resonance frequency deviation. The resonance frequency deviation, or off-resonance frequency is the frequency difference between the ejected or theoretical resonance frequency of the nuclear spins at a defined location, and the actual resonance frequency of these spins that is detected. Expressed differently, the resonance frequency deviation is the frequency difference between the resonance frequency of the spins at a defined location given an optimally homogeneous basic magnetic field, and the actual resonance frequency of these spins that result due to inhomogeneities of the basic magnetic field.
In order to acquire all information belonging to the slice in spite of the distortions of that excited slice, according to the prior art it is known to additionally implement a phase coding in the slice selection direction, for example as is described in US 2010/0033179 A1 and in “SEMAC: Slice Encoding for Metal Artifact Correction in MRI”, W. Lu et al., Magn. Res. in Med. 62: (2009), Pages 66-76.
The primary disadvantage of these known methods is in that the measurement time is multiple times longer since every slice is essentially scanned multiple times in comparison to a method without phase coding in the slice selection direction. The factor by which the measurement time increases corresponds to the number of phase coding steps in the slice selection direction.
Having knowledge of the resonance frequency deviation for each slice would enable the phase coding steps to be individually adapted for each slice in the slice selection direction, such that only exactly as many phase coding steps are implemented as are necessary to completely correct the distortions due to the inhomogeneities of the basic magnetic field.
For individual adaptation of the phase coding steps for each slice, for example, there exists a method as is described on Page 3083 in “Adaptive Slice Encoding for Metal Artifact Correction”, B. A. Hargreaves et al., Proc. Intl. Soc. Mag. Reson. Med. 18 (2010). In this known method, however, information about high off-resonances is lost due to technical limitations since the signal is completely dephased before the beginning of the readout.
Another approach is described on Page 3169 in “Distortion Scout in Metal Implants Imaging”, G. Li et al., Proc. Intl. Soc. Mag. Reson. Med. 19 (2011). In a turbo spin echo sequence, the readout gradient is thereby executed on the same spatial axis as the slice selection gradient. The total distortion or total shift of the slice which results due to the slice selection gradient and the readout gradient can be detected, and from this the resonance frequency deviation can be calculated, and the number of phase coding steps for each slice can be adapted individually.
However, in specific cases this known method detects no distortion of the slice in spite of strong off-resonances, and therefore determines a false resonance frequency deviation. Moreover, in this method the resonance frequency deviation in a single measurement can be determined only in the precision of the excitation bandwidth.